oracle with子句

最新推荐文章于 2025-04-14 23:52:20 发布

zhenghun

最新推荐文章于 2025-04-14 23:52:20 发布

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以例子学习with：

[sql] view plain copy print ?

with
--查询部门和部门的总薪水
dept_costs as (
select d.department_name,sum(e.salary) dept_total
from departments d,employees e
where d.department_id=e.department_id
group by d.department_name
),
--利用上一个with查询的结果，计算部门的平均总薪水
avg_costs as (
select avg(dept_total) dept_avg
from dept_costs
)
--从两个with查询中比较并且输出查询结果
select *
from dept_costs
where dept_total > (select dept_avg from avg_costs)
order by department_name

with
--查询部门和部门的总薪水
  dept_costs as (
                 select d.department_name,sum(e.salary) dept_total
                   from departments d,employees e
                  where d.department_id=e.department_id
                  group by d.department_name
                 ),
--利用上一个with查询的结果，计算部门的平均总薪水
  avg_costs as (
                select avg(dept_total) dept_avg
                  from dept_costs
                )
--从两个with查询中比较并且输出查询结果
  select *
    from dept_costs
   where dept_total > (select dept_avg from avg_costs)
  order by department_name

          注释：
          ① 子查询可重用相同或者前一个with查询块，通过select调用（with子句也只能被select调用）
          ② with子句的查询输出存储到用户临时表空间，一次查询，到处使用
          ③ 同级select前有多个查询定义，第一个用with，后面的不用with，并且用逗号分割
          ④ 最后一个with查询块与下面的select调用之间不能用逗号分割，只通过右括号分离，with子句的查询必须括号括起
          ⑤ 如果定义了with子句，而在查询中不使用，则会报ora-32035错误，只要后面有引用的即可，不一定在select调用，在后with查询块引用也是可以的
          ⑥ 前面的with子句定义的查询在后面的with子句中可以使用，但是一个with子句内部不能嵌套with子句
          ⑦ with查询的结果列有别名，引用时候必须使用别名或者*

再来看with的语法

㈠ as和select中的括号不能省略
㈡同级别select调用，with只能定义一次，多个用逗号分隔，但最后一个with子查询与下面的实际查询之间没有逗号

with子句的优点

           ① with子句有可能会改变执行计划
           ② with子查询只执行一次，将结果存储在用户的临时表空间，可多次引用，增加性能
           ③ sql的可读性较强

案例：

Ⅰ一般使用方式

[sql] view plain copy print ?

with
--查询销售部门员工的姓名
saler_name as (
select department_id from departments where department_name='SALES' order by department_id
)
select last_name,first_name
from employees e
where department_id in (select * from saler_name)

with
--查询销售部门员工的姓名
  saler_name as (
                 select department_id from departments  where department_name='SALES' order by department_id
                 )
select last_name,first_name
  from employees e
 where department_id in (select * from saler_name)

注释：使用with子句，可以在复杂的查询中预先定义好一个结果集，然后在查询中反复使用，不使用会报错。而且with子句获得的是一个临时表，必须采用select from （with查询名）

Ⅱ 在多数子查询中引用，同级可见

[sql] view plain copy print ?

select last_name
from (with
--查询销售部门员工的姓名
saler_name as (
select department_id from departments where department_name='SALES' order by department_id
)
select last_name,first_name
from employees e
where department_id in (select * from saler_name)
)

select last_name
  from (with
--查询销售部门员工的姓名
  saler_name as (
                 select department_id from departments  where department_name='SALES' order by department_id
                 )
select last_name,first_name
  from employees e
 where department_id in (select * from saler_name)
        )

Ⅲ 在集合在引用

集操作的两个select调用被当做是同级的，不能出现两个with定义

[sql] view plain copy print ?

with
--查询销售部门员工的姓名
saler_name as (
select department_id from departments where department_name='SALES' order by department_id
)
select last_name,first_name
from employees e
where department_id in (select * from saler_name)
union all
select last_name,to_char(null)
from employees